SEASONAL INFLUENZA - TRAVELHEALTHPRO
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Seasonal Influenza
Influenza is a highly infectious, viral infection of the respiratory tract; outbreaks peak
during the winter months of the northern and southern hemispheres, and occur year
around in the tropics
Key Messages
Seasonal influenza is a viral infection that spreads from person to
person through respiratory droplets.
Seasonal influenza occurs throughout the world; outbreaks peak during
the winter months of the northern and southern hemispheres and occur
year round in the tropics.
Influenza is usually self-limiting with recovery within two to seven days;
severe illness and death can occur, particularly in those with pre-
existing medical conditions.
The three main ways to prevent seasonal influenza are good cough and
hand hygiene, vaccination, and antiviral medication.
Annual vaccination is recommended for some individuals prior to the
start of the influenza season to provide protection to those who are at
higher risk of severe illness.
In addition to vaccination, special antiviral medications may be
considered for some individuals in some circumstances either as
treatment or prevention.
Overview
Influenza is a viral infection of the respiratory tract; symptoms appear rapidly but last a relatively
short period of time. In healthy individuals, influenza is usually self-limiting (i.e. it resolves without
treatment), with recovery within two to seven days [1].
There are three types of seasonal influenza viruses, types A, B, and C [1, 2]. Influenza A and B
viruses cause outbreaks and epidemics. Only influenza type A viruses are known to have caused
pandemics (i.e. worldwide spread of a new disease) [2, 3]. Influenza type C virus usually causes
mild infections and is detected much less frequently [2].
In April 2009, an influenza A/H1N1 virus of swine origin was the cause of a global pandemic which
continued until July 2010 [4, 5]. This particular strain, influenza A(H1N1) pdm09, continues to
circulate globally as seasonal influenza [5, 6].
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The relevant strains influenza viruses to be included in seasonal influenza vaccines are determined
each year by the World Health Organization (WHO) [7].
Risk areas
Seasonal influenza occurs throughout the world. In temperate regions of the northern hemisphere
most influenza activity is from November to March (between December and March in the United
Kingdom). In the southern hemisphere most influenza activity occurs between April and September.
In the tropics, influenza viruses can circulate throughout the year.
Up to date worldwide information on seasonal influenza activity is available from the World Health
Organization (WHO). Data on seasonal influenza activity in Europe is also available from the
European Centre for Disease Prevention and Control.
Risk for travellers
Influenza has been described as the most frequent vaccine preventable infection among travellers
to tropical and subtropical countries [8-10].
The risk of exposure to influenza during travel depends on time of year, type of travel, destination
and duration. However, travellers can be at risk during the summer months at their destination,
particularly if travelling in large groups that include tourists from other regions of the world where
influenza viruses are currently circulating [11]. Transmission and spread of influenza infection is
accelerated in crowded conditions (e.g. during travel by air plane, cruise ship or when attending
mass gatherings) [8].
There is limited data on foreign travel-related cases of influenza in the UK, as travel history is not
routinely collected for seasonal influenza cases.
Transmission
Influenza virus spreads easily from person to person via respiratory droplets when coughing and
sneezing. Crowded, enclosed environments facilitate transmission [1]. Another potential route of
infection is from touching surfaces or objects contaminated with respiratory droplets and then
touching the mucous membranes on the nose or mouth [1, 2].
Signs and symptoms
Classic symptoms of influenza are the sudden onset of fever, chills, headache, cough (usually dry),
extreme fatigue, sore throat, runny nose and muscle and joint pain [1, 2]. Influenza can affect all
age groups, and the burden of disease on each age group can vary from season to season,
depending on the strains circulating.
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Although infection is usually self-limiting, it can be complicated by secondary infections (such as
bronchitis, bacterial pneumonia and ear infections). Influenza may exacerbate underlying medical
conditions leading to life threatening illness. The elderly, the very young and those with serious
medical issues (e.g. chronic (long-term) heart conditions, chronic respiratory conditions and
immunosuppression), are particularly at risk. Unusually, influenza may be complicated by
encephalitis (inflammation of the brain) and meningitis (infection of the protective membranes that
surround the brain and spinal cord) [1].
Pandemic influenza A (H1N1) pdm 09 in 2009/10 generally caused a mild disease in children and
young adults, however, severe cases and deaths did occur in these age groups. In the UK, most
deaths were in those younger than 65 years old, the majority of whom suffered from an underlying
illness, although deaths in previously healthy individuals were documented. Pregnant women were
also at higher risk of severe illness [12].
Diagnosis and treatment
During the influenza season, diagnosis is usually made by consideration of clinical signs and
symptoms; specific testing for influenza virus (laboratory diagnosis) is usually only indicated in
cases of complicated influenza [13], or for surveillance purposes.
Preventing seasonal influenza
Vaccination against influenza is the most effective way of preventing the illness [1]. See ‘Vaccine
information’ section for details on who should be offered vaccination for personal protection.
Travellers are advised to take the following precautions to reduce their risk of exposure to
respiratory infections and prevent spreading them:
Avoid close contact with symptomatic individuals
Avoid crowded conditions where possible
Practise frequent hand washing
Practise ‘cough hygiene’: sneezing or coughing into a tissue and promptly discarding it
safely, and frequent hand washing
Avoid travel if unwell with influenza-like symptoms
In the UK there are licensed antiviral drugs that can be considered (in addition to vaccination) in
certain circumstances for protection against severe influenza, or for treatment of infection for those
who are indicated; health professionals should follow:
NICE Guidance on prophylaxis (prevention) of influenza with antivirals
NICE Guidance on the treatment of influenza with antivirals [1]
Vaccine information
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In the UK, influenza vaccines are prepared using virus strains recommended annually by WHO. The
vaccine formulation is reviewed and changed as necessary to provide protection against strains of
influenza viruses that are predicted to circulate in a given season. Information on epidemiological
trends and circulating influenza viruses are gathered by WHO, to ensure the closest possible match
between circulating influenza viruses and influenza vaccines [6].
In the UK, influenza vaccines are prepared more than 6 months in advance of the northern
hemisphere winter season. The vaccines are either trivalent (containing two influenza A subtypes
and one influenza B subtype) or quadrivalent (containing two influenza A subtypes, and two
influenza B subtypes).
Most vaccines are grown in embryonated hen’s eggs. All but one influenza vaccine currently used in
the UK are inactivated and do not contain live viruses. One vaccine, the intranasal administered
vaccine (a nasal spray squirted up each nostril), contains attenuated (weakened), cold adapted
viruses, which cannot replicate at body temperature. None of the vaccines available can cause
clinical influenza in those that are vaccinated [1].
Following vaccination, protection is thought to last for approximately one year, although this may
be less for the elderly. After vaccination, antibody levels take 10 to 14 days to reach protective
levels [1].
Further information on the UK’s influenza programme, including information about vaccine virus
strains, is available from Public Health England.
Indications for use of vaccine
The aim of the UK’s influenza programme is to protect those most vulnerable to serious illness or
death if they develop influenza. The UK has also recently introduced the offer of influenza vaccines
for healthy children. Influenza vaccine becomes available annually in the UK in September/October
and is offered to [1]:
All those aged 65 years or older
All those aged six months or older in the clinical risk groups (listed below)
Children not in clinical risk groups that are eligible for vaccination as part of the ongoing
phased roll out of the programme to all children aged two to less than seventeen years old
Additional specific information for the current influenza season in the UK is available from Public
Health England: annual national flu immunisation programme plan.
Clinical risk groups
Asplenia (absent spleen) or splenic dysfunction (spleen not fully functioning)
Chronic (long-term) respiratory disease
Chronic heart disease
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Chronic renal (kidney) disease
Chronic liver disease
Chronic neurological (nervous system) disease
Diabetes
Immunosuppression (weakened immune system)
Morbidly obese (defined as having a BMI of 40 and above)
Pregnant women
In addition, vaccination is recommended for:
Those living in long stay care facilities
Health care and social care workers with direct patient or service user contact
Carers of disabled or vulnerable individuals
Household contacts of people with weakened immune systems
The list above is not exhaustive. Public Health England states that influenza vaccine can be offered
to others based on the clinical judgement of a health professional [1].
In the UK, vaccination is not routinely recommended for travellers. Health professionals should
carefully assess the risk of influenza for healthy travellers to tropical and sub-tropical regions during
the influenza season at the destination. A decision to vaccinate healthy adults could be made but
the vaccine cannot be given as an NHS service for this category of traveller. In the UK, northern
hemisphere influenza vaccines for the winter season usually become available in October. Most of
the stock is used over the winter and supplies may not be available during the spring and summer
months. Currently, the southern hemisphere vaccine (which may contain different strains of
influenza viruses as recommended by WHO) is not available in the UK.
See Immunisation against infectious disease (The 'Green book' Influenza chapter) for further
information on indications for influenza vaccine.
Vaccine schedules
The updated influenza chapter of Immunisation against infectious disease (‘Green Book’) should be
consulted for current vaccine schedules, routes of administration and dosages.
Detailed vaccine information can also be found in the manufacturer’s Summary of Product
Characteristics (SPC) on the electronic Medicines Compendium (eMC).
Contraindications
Very few individuals are unable to receive any influenza vaccine. As with all vaccines, anyone with a
moderate to severe acute febrile illness should delay vaccination until they have recovered.
The vaccine should not be given to anyone with a confirmed anaphylactic reaction to a previous
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dose of the vaccine, or to any component of the vaccine [1]. However, the Joint Committee on
Vaccines and Immunisation (JCVI) has advised that children with an egg allergy – including those
with previous anaphylaxis to egg – can be safely vaccinated with the live attenuated influenza
vaccine (LAIV) in any setting (including primary care and schools). The only exception is for children
who have required admission to intensive care for a previous severe anaphylaxis to egg, for whom
no data are available; such children are best given LAIV in the hospital setting (see PHE ‘green
book' chapter for full details) [1].
Most influenza vaccines in the UK are prepared from viruses grown in embryonated hen’s eggs.
Influenza vaccines with egg albumin content greater than 0.12 μg/ml, or where the content is not
stated, should not be given to egg allergic individuals. Some influenza vaccines are either ‘egg free’
or have very low ovalbumin content. The ovalbumin content of available influenza vaccines and
guidance on their use for individuals with egg allergy is available from Public Health England [1].
LAIV is not recommended for children and adolescents currently experiencing an acute
exacerbation of severe asthma or active wheezing and/or needed additional bronchodilator
treatment in the previous 72 hours [1]. There are limited safety data in children who require regular
oral steroids for maintenance of asthma control, or have previously required intensive care for
asthma exacerbation – such children should only be given LAIV on the advice of their specialist and
an alternative inactivated influenza should be offered where appropriate [1].
The LAIV should not be given to immunosuppressed individuals. Pregnant women should be offered
an inactivated vaccine.
Practitioners should refer to the ‘Green Book’ influenza chapter for further guidance and seek
specialist advice if appropriate.
Adverse events
Transient reactions such as soreness, swelling or redness at the site of injection can occur. Fever,
malaise and other systemic symptoms are also reported [1].
Following administration of LAIV, symptoms such as nasal congestion and a runny nose, headache
and weakness occur commonly [1].
Rarely, other more serious adverse events (i.e. convulsions, nerve pain and a temporary low
platelet count in the blood) have been reported following influenza vaccination. However, no
relationship between vaccination and these adverse events has been established [1]. Please refer
to the ‘Green Book’ Influenza chapter for detailed information.
Allergic reactions such as angioedema (swelling of the lower layer of skin and tissue just under the
skin or mucous membranes), bronchospasm (swelling and narrowing of breathing tubes leading to
difficulties with breathing) and urticaria (hives), or full anaphylaxis (where these symptoms occur
together very quickly and are life threatening) can rarely occur, and are usually due to
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hypersensitivity to egg protein [1].
Resources
Public Health England, ‘Green book’ Immunisation against infectious disease Chapter 19
Influenza
Public Health England: National flu immunisation programme plan
Public Health England: Influenza: treatment and prophylaxis using anti-viral agents
Public Health England: Influenza vaccine - Ovalbumin content 2018-19 season
World Health Organization: Influenza
REFERENCES
1. Public Health England, Immunisation against infectious disease Chapter 19 Influenza
(updated 23 April 2019). [Accessed 17 May 2019]
2. World Health Organization Fact Sheet: Influenza (seasonal) Updated 31 January 2018
[Accessed 9 October 2018]
3. World Health Organization, What is a Pandemic? 24 February 2010. [Accessed 9 October
2018]
4. World Health Organization. Transcript of statement by Margaret Chan, Director-General of
the World Health Organization. 11 June 2009. [Accessed 9 October 2018]
5. World Health Organization. H1N1 in post-pandemic phase. 10 August 2010. [Accessed 9
October 2018]
6. World Health Organization. Influenza Update N° 325. 1 October 2018. [Accessed 9 October
2018]
7. World Health Organization. Recommendations on the composition of influenza virus
vaccines. [Accessed 9 October 2018]
8. Goeijenbier M. van Genderen, P. Ward, B. J. Wilder-Smith, A. Steffen R. Osterhaus, A. D. M.
E. Travellers and influenza: risks and prevention. J.Trav. Med. 2016, 1–10
9. Boggild AK, Castelli T, Gautret P et al. Vaccine preventable diseases in returned
international travelers: results from the GeoSentinel Surveillance Network.
28;7,389-95,2010
10. Steffen R, Behrens RH, Hill DR. et al. Vaccine-preventable travel health risks: what is the
evidence—what are the gaps? J Travel Med 2015; 22: 1–12. [Accessed 9 October 2018]
11. Regan J.J., Tardivel K., Lippold S.A., Kornylo Duong K. Cruise ship travel. In: US Centers for
Disease Control and Prevention. Health Information for International Travel 2018, Chapter 6
[Accessed 9 October 2018]
12. Pebody RG, McLean E, Zhao H, et al. Pandemic Influenza A (H1N1) 2009 and mortality in the
United Kingdom: risk factors for death, April 2009 to March 2010. Euro Surveill
2010;15:pii=19571 [Accessed 9 October 2018]
13. Public Health England. Guidance: Referral of influenza samples to Respiratory Virus Unit,
PHE Colindale. [Accessed 9 October 2018]
Published Date: 10 Oct 2018
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Updated Date: 17 May 2019
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